1. Field of the Invention
The present invention relates to a transmission apparatus (or an NE, which is an abbreviation of Network Element). More particularly, the present invention relates to control to switch a line in the event of a line failure.
2. Description of the Related Art
A variety of switching techniques adopted at the present time to relieve a currently used line from a failure are defined in specifications of a SONET/SDH standard, which is referred to as the SONET standard instead of the SONET/SDH standard. Consider processing to transport a packet in accordance with the SONET standard. Originally, a SONET network is assumed to be a network operated for point-to-point communications of the connection type. For this reason, information flowing through a line in the network is treated equally without distinguishing the priority level of one information from that other information regardless of whether the information is audio information or packet information. Connection information operates in communication of a connectionless type till the information is accommodated in a SONET apparatus. Connection information operating in communication of a connectionless type can be moved in communication of a connection type, which permanently assigns channels for flowing information. If information is moved in communication of a connection type, the communication in which information does naturally flow all the time is converted into a communication for flowing information continuously in a forcible manner. If one considers the fact that the utilization rate of a communication line decreases due to a continuous flow of information, one will know that such a forced continuous flow of information is wasteful.
As a spare line to be used as a relief line of a presently used line in accordance with the construction of the SONET standard, the same line as the presently used line needs to be prepared. If the act to relieve a presently used line by using a spare line is applied to a communication of the connectionless type, the waste of the presently used line used in a communication converted into the connection type is incurred by the spare line as it is. As a matter of fact, the spare line itself is, to the bitter end, a spare line, which is an inefficient line used only in the event of a line failure. During a communication of packet information, not only is the spare line itself inefficient, but the presently used line to be relieved by the spare line is also inefficient. Conventional techniques for relieving a presently used line include APS (Automatic Protection Switching) (1+1), APS (1:N), BLSR (Bidirectional Line Switched Ring), and so on.
FIG. 25 is a diagram showing a change taking place in the event of a line failure with the conventional APS (1+1) technique adopted. As shown in FIG. 25, spare lines 4P#1 and 4P#2 having the same band as the presently used lines 2W#1 and 2W#2 between an NE1#1 and an NE1#2 are needed. As indicated by their names, the spare lines 4P#1 and 4P#2 are each a spare line. Thus, in a normal condition, the spare lines 4P#1 and 4P#2 are not used. It is not until a line failure that is detected on the presently used lines 2W#1 and 2W#2, making it necessary to relieve the presently used lines 2W#1 and 2W#2 that the spare lines 4P#1 and 4P#2 are used.
FIG. 26 is diagrams showing a change taking place in the event of a line failure with the conventional APS (1:N) technique adopted. In the (1:N) configuration, for a plurality of pairs of presently used lines 12W#1j, 12W#2j and 12W#3j between transmission apparatus NE10# and NE10#2 where j=1 and 2, only a pair of spare lines 12P#j where j=1 and 2 is provided. Thus, if failures occur on more than one pair of presently used lines, only one pair can be relieved. FIG. 26 shows typical failing line relieving control executed in the manner of granting priority to the earlier. Assume that a line failure is detected on the presently used lines 12W#11 and 12W#12. In this case, the presently used lines 12W#11 and 12W#12 are replaced by the spare lines 12P#1 and 12P#2 as shown in the middle diagram of FIG. 26. Assume that a line failure thereafter is detected on the presently used lines 12W#21 and 12W#22. In this case, the presently used lines 12W#21 and 12W#22 cannot be relieved as shown in the bottom diagram of FIG. 26.
FIG. 27 is a block diagram showing a configuration adopting the APS (1:2) technique. As shown in FIG. 27, transmission apparatus NE10#i where i=1 and 2 each include ETHER interface units 13#ij where j=1 and 2, a line exchange unit 14#i, presently used OC12 interface units 15#ij where j=1 and 2, and spare OC12 interface units 15#iP. The ETHER interface units 13#ij are typical interface units for interfacing with respect to data, LAN data, data frames and data packets. Spare OC12 interface units 15#iP where i=1 and 2 are in an available state of being not connected to the cards of the ETHER interface units for terminating signals of a terminal. The ETHER interface units 13#ij inputs a packet from a data network such as an ETHER network, maps the packet onto a synchronous frame such as the STS1X 12 synchronous frame and passes on the frame to the line exchange unit 14#i. The line exchange unit 14#i carries out line exchange processing between the ETHER interface units 13#ij where j=1 and 2 and the OC12 interface units 15#ij, 15#iP where j=1 and 2. The presently used OC12 interface units 15#ij and the spare OC12 interface units 15#iP exchange an OC12 packet with the SONET network.
FIG. 28 is a block diagram showing a normal state of the configuration adopting the APS (1:2) technique. As shown in FIG. 28, ETHER termination units 16#ij where j=1 and 2 receive a packet from an asynchronous network and outputs the packet to a SONET mapping unit 17#1j. The SONET mapping unit 17#1j maps the packet onto an STS12×12 synchronous frame. The STS1X 12 synchronous frame is output to the OC12 interface units 15#1j via the line exchange unit 14#1. Multiplexed in an OC12 packet, the STS12×12 synchronous frame is received by the OC12 interface units 15#2j. In this way, the communications between the ETHER interface units 13#1j where j=1 and 2 and the ETHER interface units 13#2j where j=1 and 2 are implemented through the presently used OC12 interface units 15#ij where i=1 and 2 whereas j=1 and 2. At that time, the spare OC12 interface units 15#iP where i=1 and 2 are not functioning.
FIG. 29 is a diagram showing an internal state of the configuration adopting the APS (1:2) technique in the event of a line failure. Assume that a line failure is detected on the presently used line between the presently used OC12 interface units 15#11 and 15#21. In this case, the communication between the ETHER interface units 13#11 and 13#21 is implemented by the spare OC12 interface units 15P#1 and 15P#2.
FIG. 30 is diagrams showing a change taking place in the event of a line failure with the conventional BLSR (NUT+PCA) technique adopted. The PCA is an abbreviation of Protection Channel Access, which is the name of a technology of using a spare side as an unredundant line in order to increase the efficiency of the line utilization. On the other hand, the NUT is an abbreviation of Non-preemptible Unprotected Traffic, which is a line operated as an unredundant line. The NUT is operated as a presently used line of the BLSR technique. By specifying the presently used side as a NUT line and specifying the spare side as a PCA line, it is possible to provide a configuration requiring no line switching even in the event of a line failure occurring on either side. In this case, since the spare side is operated as a PCA line while the presently used side is in a state of being guarded by a NUT line, impossibility of utilization as a relieved line on the presently used side is indicated.
If a PCA line is specified as described above, an independent signal can be flown also to the spare line. By merely specifying a PCA line, in order to relieve a presently used line in the event of a line failure occurring on the presently used line, the independent signal flowing to the PCA line is cut off. If the presently used line is specified as a NUT line, on the other hand, a spare line cannot be switched in even if the presently used line fails. By specifying the presently used side as a NUT line and specifying the spare side as a PCA line, an independent signal can be flown to the spare line and, in addition, no such crushing will occur even in the event of a line failure. However, the presently used line cannot be relieved.
The NEs 20#1, 20#2, 20#3 and 20#4 shown in FIG. 30 form a ring network. A signal from the NE 20#2 reaches the NE 20#4 through line (1) connecting the NE 20#2 to the NE 20#1 and line (2) connecting the NE 20#1 to the NE 20#4. At that time, assume that line (2) is operated as a NUT line, a line failure is detected on line (2) and the BLSR function works. In this case, if the transmission lines are used normally, a signal from the NE 20#2 reaches the NE 20#4 through lines (1), (3), (4) and (5). Since line (3) is operated as a PCA line, however, line (2) operated as a NUT line cannot be relieved.
FIG. 31 is a block diagram showing the conventional BLSR switching technique. As shown in FIG. 31, the NE 20#i has ETHER interface units 30#ij where j=1 and 2, a line exchange unit 32#i, an OC48 east 34#iE and an OC48 west 34#iW. With respect to the NE 20 #1, route (1) includes a line between the ETHER interface unit 30#11 and the OC48 west 34#1W, a line between the OC48 west 34#1W and the OC48 east 34#2E and a line between the OC48 east 34#2E and the ETHER interface unit 30#21. Route (2) includes a line between the ETHER interface unit 30#12 and the OC48 east 34#1E, a line between the OC48 east 34#1E and the OC48 west 34#3W and a line between the OC48 west 34#3W and the ETHER interface unit 30#31. Route (3) includes a line between the ETHER interface unit 30#13 and the OC48 west 34#1W, a line between the OC48 west 34#1W and the OC48 east 34#2E and a line between the OC48 east 34#2E and the ETHER interface unit 30#22.
FIG. 32 is a block diagram showing operations in a normal condition of the NE 20#1 shown in FIG. 31. The ETHER interface units 30#ij where j=1, 2 and 3 have ESTHER termination units 40#1j where j=1, 2 and 3 and SONET mapping units 42#1j where j=1, 2 and 3, respectively. The ETHER termination units 40#1j where j=1, 2 and 3 receive 1 Gbps packets from ETHER networks 50#1j respectively where j=1, 2 and 3, and output the packets to the SONET mapping units 42#1j respectively where j=1, 2 and 3. The SONET mapping units 42#1j where j=1, 2 and 3 map the 1 gbps packets onto STS1×24 frames and output the frames to the line exchange unit 32#1. The line exchange unit 32#1 outputs an STS1 frame input from the SONET mapping units 42#1j where j=1, 2 and 3 to the OC48 west 34#1W, the OC48 east 34#1E and the OC48 west 34#1W respectively in accordance with the line setting. When the OC48 west 34#1W and the OC48 east 34#1E receive STS1X 48 frames, the OC48 west 34#1W and the OC48 east 34#1E map STS1X 48 frames onto an OC48 packet and transmit the packet to a SONET network-BLSR right handed unit 60#R and a SONET network-BLSR left handed unit 60#L.
FIG. 33 is a block diagram showing operations in a normal condition of the NE 20#2 shown in FIG. 31. The OC48 east 34#2E separates an STS1X 24 frame from an OC48 packet received from the SONET network-BLSR left handed unit 60#L, and the line exchange unit 32#2 supplies the STS1X 24 frame to the ETHER interface units 30#21 and 30#22. The ETHER interface units 30#21 and 30#22 demap the STS1X 24 frame back onto a packet, which is transmitted to ETHER networks 50#21 and 50#23.
FIG. 34 is a block diagram showing operations in a normal condition of the NE 20#3 shown in FIG. 31. The OC48 west 34#3W separates a STS1X 24 frame from an OC48 packet received from the SONET network-BLSR right handed unit 60#R and the line exchange unit 32#3 supplies the STS1X 24 frame to the ETHER interface unit 30#31. The ETHER interface unit 30#31 demaps the STS1X 24 frame back onto a packet, which is transmitted to an ETHER network 50#31.
FIG. 35 is a diagram showing routes established in the event of a line failure occurring in the BLSR network. The line failure is detected on a line connecting the NE 20#1 to the NE 20#3. Since route 1 is not affected by the line failure, route 1 remains the same as the normal condition, including a line between the ETHER interface unit 30#11 and the OC48 west 34#1W, a line between the OC48 west 34#1W and the OC48 east 34#2E and a line between the OC48 east 34#2E and the ETHER interface unit 30#21. On the other hand, a signal on a line between the OC48 east 34#1E and the OC48 west 34#3W is lost from route 2. Thus, route 2 is newly established as follows to comprise a line between the ETHER interface unit 30#12 and the OC48 west 34#1W, a line between the OC48 west 34#1W and the OC48 east 34#2E and a line between the OC48 east 34#2E and the ETHER interface unit 30#31. Since route 3 is specified as a PCA route and there is no other NUT specification, no spare line is available. Thus, the route is cut off. At that time, the NEs 20#i where i=1, 2 and 3 enter the following states.
FIG. 36 is an explanatory diagram showing operations carried out by the NE 20#1 shown in FIG. 31 in the event of a line failure. As shown in FIG. 36, the line exchange unit 32#1 carries out exchange processing on a line between the ETHER interface unit 30#12 and the OC48 west 34#1W. Since a connection destination is being used by the ETHER interface unit 30#12, the ETHER interface unit 30#13 does not have a connection destination.
FIG. 37 is an explanatory diagram showing operations carried out by the NE 20#2 shown in FIG. 31 in the event of a line failure. As shown in FIG. 37, the line exchange unit 32#2 carries out exchange processing on a line between the OC48 east 34#2E experiencing exchange processing with the ETHER interface unit 30#22 and the OC48 west 34#2W. Since a connection destination is being used by the ETHER interface unit 30#12, the ETHER interface unit 30#22 does not have a connection destination.
FIG. 38 is an explanatory diagram showing operations carried out by the NE 20#3 shown in FIG. 31 in the event of a line failure. As shown in FIG. 38, the line exchange unit 32#3 carries out exchange processing on a line between the ETHER interface unit 30#31 and the OC48 east 34#3E.
As described above, in the conventional BLSR switching technique, the PCA technique treating a spare line like a presently used line is not capable of relieving a presently used line even if a line failure is detected. This is because, from the first, the PCA concept assumes that a spare line must be used for flowing a signal that must be utilized effectively and a relief operation is not taken into consideration. With this conventional technique, however, the following problem arises. In the case of the conventional technique, much like the packet over SONET technique, information propagating along a line is treated like information of a connection type in spite of the fact that the information is a connectionless type. Thus, the relief operation can be carried out only in line units as described above. As a result, the relief operation cannot be implemented in a way with a spare line used effectively.